JP2011198540A - Display apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display apparatus which can improve transmittance in a desired wavelength by using a thin silver film layer that contains 90 wt.% or more of silver and has a film thickness of thinner than 30 nm.SOLUTION: In the display apparatus having an organic EL device emitting each RGB color, an electrode on a light takeout side of the organic EL device includes a thin silver film layer 2 brought into contact with a charge transport layer 1; and a refraction index control layer 3 that is commonly placed on the organic EL device emitting each RGB color on the thin silver film layer 2 where an effective refractive index (n) represented by an expression: n=0.7 n+0.3 nis 1.4 or more and 2.3 or less. In the expression, nis the refraction index of the refraction index control layer 3 and nis the refraction index of the charge transport layer 1.

Description

  The present invention relates to a display device in which a plurality of organic EL elements using a thin film containing 90% by weight or more of silver as an electrode on the light extraction side are arranged.

  The organic EL element has a structure in which an organic compound layer including a light emitting layer is sandwiched between an anode and a cathode, and at least one of the light extraction side electrodes needs to be transparent. As the transparent electrode, for example, an oxide transparent electrode such as indium tin oxide (ITO) is generally used.

  In addition, as a transparent electrode, use of a metal thin film has been attempted for the purpose of achieving higher transmittance and conductivity. Examples of the material for the metal thin film include gold, silver, and copper that have high electrical conductivity and high transmittance in the visible light region. Among these, silver is promising because the wavelength dependency of transmittance is flat and the conductivity is high. However, even if silver is thinned, the transmittance is not simply improved. One of the causes is the localized surface plasmon absorption of silver (see Non-Patent Document 1).

  Generally, when the film thickness of the silver thin film is about 10 nm for improving the transmittance, the thin film is not a continuous uniform film but a discontinuous island film. Since the island-shaped silver particles have localized surface plasmon absorption in the visible light region, there arises a problem that the transmittance of the silver thin film is reduced.

  As a countermeasure, by providing an underlayer made of a metal other than silver below the thickness of the silver thin film, a continuous uniform silver thin film is formed instead of islands. Has been proposed (see Patent Document 1).

JP 2008-171737 A

J. et al. J. et al. Mock, D.C. R. Smith, S.M. By Schultz, “Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles”, Nano Letters, 2003, Vol. 3, no. 4, p. 485-491

  However, when a base layer made of metal is provided as in Patent Document 1, there is not a little transmittance loss derived from the base layer even though the thickness of the base layer is less than or equal to the thickness of the silver thin film. . In addition, since the underlayer assumes the silver film formation control function of controlling the formation of the silver thin film island, only a metal having this function can be selected as the underlayer.

  In view of the above problems, the present invention provides a display device in which organic EL elements using a thin film containing 90% by weight or more of silver as an electrode on the light extraction side are arranged, and the transmittance at a desired wavelength can be improved. Objective.

  The configuration of the present invention made to achieve the above object is as follows.

That is, in a display device having an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue.
The organic EL device includes an anode, a cathode, and an organic compound layer including a light emitting layer and a charge transport layer between the anode and the cathode, and the electrode on the light extraction side of the anode and the cathode is A silver thin film layer that is in contact with the charge transport layer and contains 90% by weight or more of silver and has a film thickness of less than 30 nm.
On the silver thin film layer, the organic EL element that emits red, the organic EL element that emits green, and the organic EL element that emits blue, and a refractive index control layer disposed in common.
The display device is characterized in that an effective refractive index (n _eff ) represented by the following formula is 1.4 or more and 2.3 or less.

n _eff = 0.7 n _u +0.3 n _d
n _u : refractive index of refractive index control layer n _d : refractive index of charge transport layer

  According to the present invention, the silver thin film layer is formed in contact with the charge transport layer, and the refractive index control layer is formed on the silver thin film layer in common with each color organic EL element. By controlling the effective refractive index by this laminated structure, the absorption peak wavelength of localized surface plasmon absorption expressed by island-like silver particles in the silver thin film layer can be shifted. Therefore, the superimposition of the emission spectrum of the organic EL element that emits a desired color and the localized surface plasmon absorption is reduced, and an excellent effect is obtained in that the transmittance at a desired wavelength can be improved.

It is a schematic diagram which shows the cross-sectional structure of the three layers of the light extraction side of the organic EL element of this invention. It is a microscope picture which shows the observation result by the scanning electron microscope (SEM) of a quartz glass substrate / silver thin film. It is explanatory drawing which shows the visible-ultraviolet light absorption spectrum of a quartz glass substrate / silver thin film. It is explanatory drawing which shows the peak wavelength ((lambda) p) dependence of the localized surface plasmon absorption with respect to an effective refractive index ( _neff ). It is a schematic diagram which shows the cross-section of the organic EL element of this invention. It is explanatory drawing which shows the emission spectrum and local surface plasmon absorption from a green organic EL element without a refractive index control layer. It is explanatory drawing which shows the effective refractive index dependence of green emission spectrum integration. It is explanatory drawing which shows the emission spectrum and localized surface plasmon absorption from a red organic EL element without a refractive index control layer. It is explanatory drawing which shows the effective refractive index dependence of red emission spectrum integration. It is explanatory drawing which shows the emission spectrum and local surface plasmon absorption from a blue organic EL element without a refractive index control layer. It is explanatory drawing which shows the effective refractive index dependence of blue emission spectrum integration. It is a schematic diagram which shows the cross-sectional structure of the display apparatus of this invention. It is explanatory drawing which shows the emission spectrum and localized surface plasmon absorption from a display apparatus without a refractive index control layer.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments.

  First, the laminated structure on the light extraction side of the organic EL element of the present invention will be described with reference to FIG. FIG. 1 is a schematic view showing a cross-sectional structure of three layers on the light extraction side of the organic EL element of the present invention. As shown in FIG. 1, the light extraction side of the organic EL device of the present invention has a structure in which a silver thin film layer 2 is formed in contact with the charge transport layer 1 and a refractive index control layer 3 is further laminated thereon. Have.

  The charge transport layer 1 may be any material that can withstand the film formation of the silver thin film layer 2 containing 90% by weight or more of silver.

  The silver thin film layer 2 is a thin film containing 90% by weight or more of silver, and is formed using a general thin film forming method such as a vacuum deposition method or a sputtering method, for example. The silver thin film layer may contain a trace amount (less than 10% by weight) of Pd, Cu, Mg, Au, and the like. The film thickness of the silver thin film layer 2 is preferably less than 30 nm, and more preferably in the range of 5.0 nm to 20 nm. If the film thickness is 30 nm or more, the local surface plasmon absorption of the silver particles is reduced, but the reflectance of the silver thin film layer 2 itself is increased and the transmittance is deteriorated.

  The refractive index control layer 3 may be formed of either an organic material or an inorganic material. Specific materials will be described later.

  Next, localized surface plasmon absorption when a silver thin film is used as an electrode on the light extraction side will be described with reference to FIGS. Here, a sample in which a quartz glass substrate, a silver thin film as the silver thin film layer 2, and a plurality of materials having different refractive indexes as the refractive index control layer 3 were selected instead of the charge transport layer 1 was prepared.

  First, the quartz glass substrate was ultrasonically washed successively with water, acetone and isopropyl alcohol, then boiled and washed with isopropyl alcohol and then dried. Further, the washed quartz glass substrate was treated with UV ozone. Next, a silver thin film layer 2 having a thickness of 10 nm was formed on a quartz glass substrate by a vacuum deposition method.

  FIG. 2 is a photomicrograph showing the observation result of the quartz glass substrate / silver thin film prepared as described above by a scanning electron microscope (SEM). As shown in FIG. 2, it is observed that the silver thin film consists of island-like silver particles.

  FIG. 3 is an explanatory diagram showing the visible-ultraviolet light absorption spectrum of the quartz glass substrate / silver thin film. In addition, Ubest V-560 (made by JASCO Corporation) was used for the measurement of a visible-ultraviolet light absorption spectrum.

  As shown in FIG. 3, in the case of air without the refractive index control layer 3, localized surface plasmon absorption of a silver thin film having a peak wavelength of 515 nm can be confirmed. This absorption reduces the transmittance when the silver thin film is used as an electrode on the light extraction side. Therefore, it is necessary to shift the peak wavelength in order to improve the transmittance at a desired wavelength.

  Therefore, in order to shift the wavelength of localized surface plasmon absorption, a refractive index control layer 3 made of a material having various refractive indexes was formed on the silver thin film layer 2. As the refractive index control layer 3, silicon dioxide (1.45), aluminum oxide (1.76), zinc oxide (1.95), indium-tin oxide (2.1), and titanium oxide (2.71) are used. In this case, each material was deposited with a film thickness of 20 nm by sputtering. When lithium fluoride (1.39) was used, a lithium fluoride layer having a thickness of 20 nm was formed by vacuum deposition. The numerical value in parentheses following the material name is the refractive index of each material.

  Further, since the refractive index control layer 3 has a function different from that of the optical interference layer used in the organic EL element in order to improve the light extraction efficiency, it is not necessary to determine the film thickness to match the emission wavelength. Furthermore, since the localized surface plasmon absorption is affected by the local environment around the silver particles, a thin film having a thickness of 30 nm or less is sufficient. Preferably, the film thickness of the refractive index control layer 3 is 10 nm or more and 20 nm or less.

Next, FIG. 4 is an explanatory diagram showing the dependence of the localized surface plasmon absorption on the peak wavelength (λp) with respect to the effective refractive index (n _eff ).

Here, the effective refractive index (n _eff ) is a value considering the refractive indexes of both the charge transport layer 1 (in the above sample, a quartz glass substrate) surrounding the silver thin film layer 2 and the refractive index control layer 3. It is shown by the formula.

n _eff = αn _u + (1−α) n _d
n _u : refractive index of refractive index control layer 3 n _d : refractive index of charge transport layer (in the above sample, refractive index of quartz glass substrate)
α: Weight factor

  Note that the weighting factor (α) is a covering rate at which the refractive index control layer 3 covers the silver lump of the silver thin film layer 2, and α = 0.7 in the present invention.

  As shown in FIG. 4, as the effective refractive index increases, the localized surface plasmon absorption of the silver thin film shifts to the longer wavelength side. This shows that the peak wavelength of localized surface plasmon absorption can be controlled by the refractive index of the peripheral layer of the silver thin film layer 2.

  Therefore, when a silver thin film is used as an electrode on the light extraction side, it is only necessary to minimize the overlap between the spectrum of light to be transmitted and the spectrum of localized surface plasmon absorption. In other words, if the refractive index of the refractive index control layer 3 is adjusted so that the peak wavelength of localized surface plasmon absorption is shifted from the spectral peak wavelength of transmitted light toward the short wavelength side or the long wavelength side, the transmittance can be obtained. An improvement effect can be obtained.

[Organic EL device]
Next, the organic EL element of the present invention will be described.

  The compounds used in this example are shown below.

<Green organic EL element>
First, the organic EL element that emits green light according to the present invention will be described with reference to FIG. FIG. 5 is a schematic diagram showing a cross-sectional structure of the organic EL element of the present invention.

  As shown in FIG. 5, the organic EL device of the present invention includes an organic compound layer including a light emitting layer and a charge transport layer between an anode and a cathode, and an anode electrode (anode) 4 and a hole injection in order from the bottom. The layer 5, the light emitting layer 6, the electron transport layer 1, and the silver thin film layer 2 and the refractive index control layer 3 as a cathode are laminated. In this element structure, the upper side is the light extraction side, and the electron transport layer 1 is in contact with the silver thin film layer 2. That is, the configuration of the electron transport layer 1 / silver thin film layer 2 / refractive index control layer 3 corresponds to the laminated structure of FIG.

  As the anode electrode 4, an aluminum neodymium thin film formed of indium zinc oxide (IZO) with a film thickness of 38 nm by a sputtering method was used. This was ultrasonically washed successively with acetone and isopropyl alcohol, then boiled and washed with isopropyl alcohol and then dried. Further, the washed transparent conductive support substrate subjected to UV ozone treatment was used in the next film forming step.

Compound (1) was used for the hole injection layer 5. Using this compound (1), a hole transport layer 5 having a thickness of 150 nm was formed by vacuum deposition. In the light emitting layer 6, the compound (2) was used as a main component, and the compound (3) was used as a light emitting dopant. Using these compounds, a light emitting layer 6 having a thickness of 30 nm was formed by simultaneous vapor deposition from different boats by a vacuum vapor deposition method. The concentration of the compound (3) was 2 wt%. Further, the compound (4) was used as the electron transport layer 1. Using this compound, an electron transport layer 1 having a thickness of 40 nm was formed by vacuum deposition. The degree of vacuum at the time of vapor deposition of the compounds shown in these compounds (1) to (4) was 5.0 × 10 ⁻⁵ Pa, and the film formation rate was 0.1 nm / sec to 0.3 nm / sec.

  Next, silver was used as a material for the silver thin film layer 2. On the electron transport layer 1, a silver thin film layer 2 having a thickness of 10 nm was formed by vacuum deposition.

  Furthermore, as the refractive index control layer 3, elements for forming silicon dioxide, aluminum oxide, zinc oxide, indium tin oxide (ITO), titanium oxide, lithium fluoride, and the compound (4) were formed. In the case of silicon dioxide, aluminum oxide, zinc oxide, indium tin oxide (ITO), and titanium oxide, each material was formed with a film thickness of 20 nm by a sputtering method. In the case of lithium fluoride or the above compound (4), each material was formed into a film with a thickness of 20 nm by a vacuum deposition method. An element without the refractive index control layer 3 was prepared as a comparative sample.

  The obtained organic EL device was covered with a protective glass tube under a dry nitrogen atmosphere and sealed with an acrylic resin adhesive so that the device was not deteriorated by exposure to air.

  FIG. 6 is an explanatory diagram showing an emission spectrum and localized surface plasmon absorption from a green organic EL element without a refractive index control layer. The horizontal axis represents the wavelength, the left vertical axis represents the EL emission intensity, and the right vertical axis represents the absorbance of the localized surface plasmon absorption.

  As shown in FIG. 6, when a DC voltage of 10 V is applied with the anode electrode as the positive electrode and the silver thin film as the negative electrode, green light emission derived from the compound (3), which is a luminescent dopant, is observed at a maximum emission wavelength of 520 nm. It was done. On the other hand, the peak wavelength of localized surface plasmon absorption is 522 nm. That is, the obtained emission spectrum largely overlaps with the localized surface plasmon absorption, and the emission intensity is reduced by the silver thin film. Therefore, the superposition of the green light spectrum and the localized surface plasmon absorption spectrum derived from the silver thin film should be minimized. That is, the localized surface plasmon absorption may be shifted by the refractive index control layer.

  FIG. 7 is an explanatory diagram showing the effective refractive index dependence of the green emission spectrum integration. Note that the emission spectrum integration is standardized with 1 when the refractive index control layer is air and the electron transport layer 1 is the compound (4) (effective refractive index 1.21). In FIG. 7, the effective refractive index is changed by changing the material of the refractive index control layer, that is, the refractive index of the refractive index control layer. As shown in FIG. 4, the localized surface plasmon absorption shifts to the longer wavelength side as the effective refractive index increases. As a result, the overlapping portion of the emission spectrum and localized surface plasmon absorption decreased, the transmittance for green emission was improved, and the emission spectrum integral increased with the effective refractive index as shown in FIG.

  As a result, in order to construct a green organic EL element with improved transmittance using a silver thin film as an electrode on the light extraction side, the refractive index of the refractive index control layer 3 is adjusted so that the effective refractive index is 1.4 or more. It can be seen that the rate should be adjusted.

<Red organic EL element>
Next, the organic EL element that emits red light according to the present invention will be described. The basic structure of the red organic EL device of the present invention is the same as that of the green organic EL device except that the compound (5) is used as the main component of the light emitting layer 6 and the compound (6) is used as the light emitting dopant. The concentration of the compound (6) was 2 wt%.

  FIG. 8 is an explanatory diagram showing an emission spectrum and localized surface plasmon absorption from a red organic EL element without a refractive index control layer. As shown in FIG. 8, when a DC voltage of 10 V is applied with the anode electrode as the positive electrode and the silver thin film as the negative electrode, red light emission derived from the compound (6), which is a luminescent dopant, is observed at a maximum emission wavelength of 630 nm. It was done. On the other hand, the peak wavelength of localized surface plasmon absorption is 522 nm. That is, the obtained emission spectrum is superimposed on the long-wavelength side of localized surface plasmon absorption, and although the amount of superposition is small compared to the green color in Example 1, the emission intensity is reduced by the silver thin film. Yes.

  FIG. 9 is an explanatory diagram showing the effective refractive index dependence of the red emission spectrum integration. Note that the emission spectrum integration is standardized with 1 when the air is without a refractive index control layer and when the electron transport layer 1 is the compound (4) (effective refractive index 1.21). As shown in FIG. 4, the localized surface plasmon absorption shifts to the longer wavelength side as the effective refractive index increases. Therefore, on the contrary to the green color, the overlapping portion of the emission spectrum and the localized surface plasmon absorption increased, and the transmittance for red light emission deteriorated. On the other hand, as the effective refractive index decreases, the localized surface plasmon absorption shifts to the short wavelength side, so that the transmittance for red light emission is improved, and the emission spectrum integration is increased as shown in FIG.

  As a result, in order to construct a red organic EL device with improved transmittance using a silver thin film as an electrode on the light extraction side, the refractive index of the refractive index control layer is set so that the effective refractive index is 2.3 or less. It can be seen that adjustment should be made.

<Blue organic EL element>
Further, the organic EL element that emits blue light according to the present invention will be described. The basic structure of the blue organic EL device of this embodiment is the same as that of the green organic EL device except that the compound (7) is used as the main component of the light emitting layer 6 and the compound (8) is used as the light emitting dopant. The concentration of the compound (8) was 2 wt%.

  FIG. 10 is an explanatory diagram showing an emission spectrum and localized surface plasmon absorption from a blue organic EL element without a refractive index control layer. As shown in FIG. 10, when a DC voltage of 10 V is applied with the anode electrode as the positive electrode and the silver thin film as the negative electrode, blue light emission derived from the compound (8), which is a luminescent dopant, is observed at a maximum emission wavelength of 490 nm. It was done. On the other hand, the peak wavelength of localized surface plasmon absorption is 522 nm. That is, the obtained emission spectrum overlaps with the short wavelength side skirt of the localized surface plasmon absorption, and the light emission intensity is reduced by the silver thin film although the overlap is small compared with the green organic EL element. .

  FIG. 11 is an explanatory diagram showing the effective refractive index dependence of the blue emission spectrum integration. Note that the emission spectrum integration is standardized with 1 when the air is without a refractive index control layer and when the electron transport layer 1 is the compound (4) (effective refractive index 1.21). As shown in FIG. 4, the localized surface plasmon absorption shifts to the longer wavelength side as the effective refractive index increases. As a result, the overlapping portion of the emission spectrum and the localized surface plasmon absorption decreased, the transmittance for blue emission was improved, and the emission spectrum integral increased with the effective refractive index as shown in FIG.

  As a result, in order to construct a blue organic EL device with improved transmittance using a silver thin film as an electrode on the light extraction side, the refractive index of the refractive index control layer is set so that the effective refractive index is 1.4 or more. It can be seen that adjustment should be made.

[Display device]
Next, with reference to FIG. 12 and FIG. 13, a display device of the present invention in which a plurality of organic EL elements of red, green and blue (RGB three colors) are arranged will be described. The structures of the arranged red, green, and blue organic EL elements and the compounds used are the same as those of the organic EL elements described above.

  FIG. 12 is a schematic view showing a cross-sectional structure of the display device of the present invention.

  As shown in FIG. 12, the anode electrode 4 and the hole injection layer 5 are laminated on the substrate 10. Next, the red light emitting layer 13, the green light emitting layer 14, and the blue light emitting layer 15 having different emission colors are separately applied to each pixel and mask-deposited. Finally, the electron transport layer 1, the silver thin film layer 2, and the refractive index control layer 3 are formed to form a display device. The film forming conditions are the same as those described for the organic EL element, but at least the refractive index control layer 3 is arranged in common for the red organic EL element, the green organic EL element, and the blue organic EL element. The film is formed.

  In the description of the organic EL element, the light to be transmitted is monochromatic light of green, red, and blue, respectively. Therefore, if the effective refractive index is monotonously increased or decreased, the localized surface plasmon absorption is monotonously shifted to the long wavelength side or the short wavelength side, respectively, and the transmittance for a desired wavelength can be improved.

  Also in the display device, it is possible to improve the transmittance by controlling the effective refractive index by the refractive index control layer 3 and the electron transport layer 1 for each color. However, in order to realize this, it is necessary to coat the refractive index control layer separately in the mask vapor deposition process in order to adjust the effective refractive index for each color, which increases the manufacturing cost. Therefore, it is cheaper to form a refractive index control layer common to red, green, and blue without coating. In the case where the refractive index control layer 3 is formed without being separately applied, the refractive index control layer 3 has an effective refractive index that reduces the overlap with localized surface plasmon absorption for all colors of red, green, and blue. It is necessary to select.

  FIG. 13 is an explanatory diagram showing an emission spectrum and localized surface plasmon absorption from a display device without a refractive index control layer. As shown in FIG. 13, the localized surface plasmon absorption has a wide absorption over visible light, but the absorption at the peak wavelength of 522 nm is particularly large. Therefore, the transmittance at a desired wavelength can be improved by shifting the peak wavelength from the spectrum peak wavelengths of red light emission, green light emission, and blue light emission.

  Ideally, if the localized surface plasmon absorption peak wavelength is shifted to a shorter wavelength than the blue emission peak wavelength of 490 nm, or is shifted to a longer wavelength side than the red emission peak wavelength of 630 nm, it is The influence of localized surface plasmon absorption can be reduced. However, the lowest refractive index material that can be used for the refractive index control layer is usually air (refractive index 1), and there is a limit to the shift toward the short wavelength side. Furthermore, the shift to the long wavelength side is similarly limited in that there are not many materials that do not absorb light in the visible light region and show a high refractive index, and titanium oxide having a refractive index of about 2.7 is considered to be the practical upper limit. Therefore, it is difficult to completely remove localized surface plasmon absorption from the visible light wavelength region.

  Therefore, it is necessary to determine the wavelength at which the transmittance should be improved in consideration of all red, green, and blue colors. In the wavelength region from the peak wavelength 520 nm of the green emission spectrum to the peak wavelength 630 nm of the red emission spectrum indicated by the arrow in FIG. That is, if the localized surface plasmon absorption peak wavelength is present in this wavelength region where the emission intensity is relatively low, the loss of light emission due to plasmon absorption is reduced.

  That is, in order to construct a display device having RGB three-color organic EL elements with improved transmittance using a silver thin film as an electrode on the light extraction side, the effective refractive index is 1.4 or more and 2.3 or less. It can be seen that the refractive index of the refractive index control layer 3 may be adjusted as described.

  The preferred embodiment of the present invention has been described above. However, this is merely an example for explaining the present invention, and various embodiments different from the above-described embodiment may be implemented without departing from the gist of the present invention. Can do. For example, in the above embodiment, the case where the electrode on the light extraction side is the cathode has been described, but the reverse configuration may be used. In that case, the hole injection layer becomes a charge transport layer in contact with the silver thin film layer.

1 charge transport layer (electron transport layer), 2 silver thin film layer, 3 refractive index control layer, 6 light emitting layer

Claims (2)

In a display device having an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue,
The organic EL device includes an anode, a cathode, and an organic compound layer including a light emitting layer and a charge transport layer between the anode and the cathode, and the electrode on the light extraction side of the anode and the cathode is A silver thin film layer that is in contact with the charge transport layer and contains 90% by weight or more of silver and has a film thickness of less than 30 nm.
On the silver thin film layer, the organic EL element that emits red, the organic EL element that emits green, and the organic EL element that emits blue, and a refractive index control layer disposed in common.
An effective refractive index (n _eff ) represented by the following formula is 1.4 or more and 2.3 or less.
n _eff = 0.7 n _u +0.3 n _d
n _u : refractive index of refractive index control layer n _d : refractive index of charge transport layer   The display device according to claim 1, wherein a film thickness of the refractive index control layer is 10 nm or more and 20 nm or less.

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